The start to July of 2012 was a hot one as a strong ridge of high pressure in the upper atmosphere allowed for a persistent feed of very warm and humid air from the south. This ridge also served to "cap" the atmosphere, preventing relief in the form of showers and thunderstorms.

Highs climbed into the 90s for the 2nd through 6th, with some locations topping 100 on the 4th and 6th. When the high humidity was factored in, afternoon and early evening heat indices ranged 100 to 115. Heat Advisories were in effect Monday for portions of the region, with Excessive Heat Warnings covering much of the Upper Mississippi River Valley from Tuesday through Friday.

Tragically, the extreme heat also was deadly as several people lost their lives from heat related factors. Five people perished from July 4th through July 10th across western and central Wisconsin, mainly from lack of air conditioning in their residences.

Factors that led to the prolonged, oppressive heat wave:

Drier than normal conditions

Strong, persistent ridge of high pressure

Little to no rain

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La Crosse

Rochester

High

Low

Average

Heat Index

July 2nd

99

73

86

105

July 3rd

99

75

87

105

July 4th

103

81

92

110

July 5th

98

79

89

111

July 6th

103

78

91

109

High

Low

Average

Heat Index

July 2nd

92

73

83

98

July 3rd

93

73

83

102

July 4th

93

75

84

104

July 5th

94

73

84

105

July 6th

96

74

85

103

Records that were broken or tied in La Crosse, WI:

July 4th: High of 103 (previous record 100 in 1911)

July 4th: Warmest low of 81 (previous record 80 in 1999). This also tied for the all-time warmest low (most recently July 13, 1995)

July 4th: Average temperature of 92 (previous record 88 in 1999). Note that this is also the 3rd warmest average temperature on record

July 5th: Warmest low temperature of 79 (previous record 77 set in 1900)

July 6th: Warmest low temperature of 78 (tied previous record set in 1886)

Numerous sites across west central Wisconsin, southeast Minnesota, and northeast Iowa saw repeated days of upper 90s to low 100s highs, with heat indices rising to the 100 to 115 range every afternoon and evening. The table below highlights some high temperatures from NWS Cooperative Observer and other automated reporting stations.